BELGRADE, Serbia — Prime Minister Aleksandar Vucic appeared headed toward a first-round victory in Serbia’s presidential election on Sunday, winning more than 50 percent of the vote among a field of 11 candidates, according to exit polls and early results.

If the preliminary vote count holds and Mr. Vucic passes the 50 percent threshold, he would avoid a riskier two-way runoff on April 16.

While Serbia is a parliamentary republic and the presidency is intended as a largely symbolic position, the actual effect of the election result is seen as removing the last check on Mr. Vucic’s power and as a further erosion of Serbia’s nascent democratic institutions.

Mr. Vucic, by far the most popular political leader in the country, will choose his successor as prime minister, most likely a pliant one, and he is expected to exercise unchallenged control over all of the country’s main political institutions: Parliament, the executive branch, the ruling party and now the presidency.

With its coalition partners, his party has a strong and solid majority in Parliament, and the courts are weak and seen as politically controlled. The departing president, Tomislav Nikolic, had been one of the few checks on Mr. Vucic’s power.

With Mr. Vucic in the president’s office, Serbia is likely to follow the same domestic and foreign policy course as during his time as prime minister: enacting the political and economic changes required for membership in the European Union, while simultaneously seeking closer relations with Russia. Creating tensions with Brussels, Mr. Vucic has refused to support sanctions against Russia.

Declaring victory in Belgrade, the capital, Mr. Vucic said, “When you have results like this, it’s clear to everyone that there is no instability,” adding, “Serbia is strong, Serbia is powerful and it will be even stronger.”

As Western governments decrease their involvement in the Balkans and membership in the European Union loses its appeal to Serbia and other countries, political leaders in the region are feeling less pressure to govern within the confines of democratic institutions or to protect human rights, press freedom and the rule of law, and to fight corruption.

The regional trend is toward “weak democracies with autocratically minded leaders, who govern through informal patronage networks and claim to provide pro-Western stability in the region,” according to a study by the Balkans in Europe Policy Advisory Group.

Observers view Mr. Vucic’s consolidation of power as a product of this drive for stability that has shaped the politics of the western Balkans over the last decade, as Western governments choose to engage with strong leaders rather than work to strengthen democratic institutions.

“Stability trumps everything,” said Jelena Milic, the director of the liberal Center for Euro-Atlantic Studies in Belgrade.

Public opinion surveys before the election showed that Serbian voters considered Mr. Vucic the best candidate for delivering stability, said Srdjan Bogosavljevic, a pollster at the Ipsos polling company in Belgrade.

Living in a region still inflamed by ethnic tensions and economic turbulence, in which older people experienced three wars in a generation, Serbs want a strong leader to guide the country, Mr. Bogosavljevic said.Mr. Vucic’s popularity surged after the arrest and prosecution of Miroslav Miskovic, one of Serbia’s wealthiest magnates, who in June was convicted of fraud and sentenced to five years in prison. Mr. Vucic had campaigned for prime minister on a promise to rein in the country’s oligarchs.

Mr. Vucic also positioned himself successfully on foreign policy: seeking good relations with Russia while also leaving no doubt that Serbia would eventually join the European Union, despite his frequent criticisms of the bloc.

“Serbian public opinion says, ‘We love Russia, but we don’t want to be part of Russia,’” Mr. Bogosavljevic said. “And we don’t like Europe, but we want to be part of Europe.”

Support for European Union membership has fallen to 47 percent, according to a poll in December by the government-run Serbian European Integration Office.

Mr. Vucic reinforced his image as an indispensable international partner during the campaign by meeting with world leaders. He met last month in Berlin with Chancellor Angela Merkel of Germany and in Moscow with President Vladimir V. Putin, who wished him “success” in the vote.

Russia has been expanding its influence in the Balkans to fill the vacuum as Western powers draw back, Ms. Milic said.

Because Mr. Vucic has delivered on some international issues important to the European Union, like encouraging moderation in Bosnia and engaging in a dialogue with the leadership of Kosovo, the bloc has refrained from overtly criticizing him for abuses like restricting press freedom.

“The E.U. is very weak and disinterested in the Balkans now, and this has enabled him to get more credit and less scrutiny for his domestic policies than he should,” said Florian Bieber, a professor of Southeast European Studies at the University of Graz in Austria.

“Behind closed doors, they always remind Vucic that he has to better protect the democratic process,” Mr. Bieber said. “But they don’t say it in public.”

A version of this article appears in print on April 3, 2017, on Page A7 of the New York edition with the headline: Serbia’s Leader Is Set to Consolidate His Power.